How do I get the value of Display -> "Scale for menu and title bars:" from the command line?

This setting is stored by dconf in /com/ubuntu/user-interface/scale-factor.

Use the command

dconf watch /com/ubuntu/user-interface/scale-factor

to watch how it changes when you move the scale. Note that, apparently, the scale factor displayed in the GUI corresponds to exactly 1/8 of this value, e.g., when I set the scale factor, via the GUI, to 1 for my primary display (on DisplayPort-1) the value /com/ubuntu/user-interface/scale-factor is set to

{'DisplayPort-0': 8, 'HDMI-0': 8, 'DVI-1': 8, 'DisplayPort-1': 8}

When I set the scale factor to 2 I obtain

{'DisplayPort-0': 8, 'HDMI-0': 8, 'DVI-1': 8, 'DisplayPort-1': 16}

When I set the scale factor to 0.5 I obtain

{'DisplayPort-0': 8, 'HDMI-0': 8, 'DVI-1': 8, 'DisplayPort-1': 4}

etc.

This is analogous for the other displays, of course. Do note that this is not one value, but an array of values, keyed with the display identifiers as returned by xrandr.

To change the setting via the command line, use something like

dconf write /com/ubuntu/user-interface/scale-factor "{'DisplayPort-0': 8, 'HDMI-0': 8, 'DVI-1': 8, 'DisplayPort-1': 8}"